We have previously characterized the long-term prognosis of 24 clinically healthy men with complete right bundle branch block (RBBB), identified from the BLSA population. Most recently we have characterized the clinical significance and prognosis of sinus bradycardia (SB) less than 50 beats/min in 47 healthy non endurance trained men older than 40 years. When compared to a control group after a mean follow-up of 5.4 years, the SB group demonstrated a higher prevalence of associated conduction abnormalities (first degree AV block, left axis deviation and complete and incomplete RBBB) 43% versus 19%, p less than .05. On maximal treadmill exercise testing, maximal heart rate did not differ between groups, although exercise duration was greater in the SB group, 11.0 plus/minus 28 versus 9.7 plus/minus 3.1 min, p less than .05. None of the subjects with SB developed syncope, high degree AV block, or other manifestations of serious cardiac conduction disturbances during follow-up. Major cardiac events (angina pectoris, myocardial infarction, congestive heart failure or cardiac death) occurred in 8% of the SB group and 11% of controls over the 5.4 year mean observation period.